1. Technical Field
The present invention relates to an electro optic device, a method of manufacturing an electro optic device, and a scanning type optical apparatus.
2. Related Art
In recent years, a scanning type image display apparatus that displays an image by raster scanning beam-shaped light, such as a laser beam, on a projected surface has been suggested. In such apparatus, completely black display may be realized by stopping supply of a laser beam. Accordingly, it becomes possible to realize a display having a higher contrast than, for example, a projector using a liquid crystal light valve. Moreover, the image display apparatus using a laser beam is characterized in that, for example, the color purity is high because the laser beam has a single wavelength and the laser beam is easily shaped (easily focused) because the laser beam has a high coherence. Accordingly, the image display apparatus using a laser beam is expected as a high-quality display that realizes high resolution and high color reproducibility. Moreover, the scanning type image display apparatus does not have fixed pixels unlike a liquid crystal display, a plasma display, and the like. Accordingly, since there is no concept of the number of pixels, the scanning type image display apparatus is also advantageous in that the resolution is easily converted.
In order to generate an image in the scanning type image display apparatus, it is necessary to scan light in a two-dimensional manner using scanners, such as a polygon mirror and a galvano mirror. There is a method of scanning light in a two-dimensional manner by causing one scanner to swing in two directions of horizontal and vertical directions. In this case, however, there is a problem that the configuration and control of a scanning system become complicated. For this reason, there has been proposed a scanning type image display apparatus in which a pair of scanners, each of which scans light in a one-dimensional manner, are provided such that the scanners perform horizontal scanning and vertical scanning, respectively. In the related art, it is common to use a polygon mirror and a galvano mirror as both scanners. A projection apparatus that uses rotary polygon mirrors as both scanners is disclosed in JP-A-01-245780.
However, even though the apparatus using the polygon mirror is disclosed in JP-A-01-245780, there is a limit to the polygon mirror or the galvano mirror since the scanning frequency rises as an image format requests a high resolution. For this reason, a system in which an MEMS (Micro Electro Mechanical Systems) technique is applied to a high-speed scanner has been recently announced. The scanner (hereinafter, referred to as an MEMS scanner) that uses the MEMS technique is manufactured using microfabrication technology for semiconductor materials, such as silicon. In this case, a mirror supported by a torsion spring or the like is driven by an electrostatic force or the like. The scanner can scan light by causing the mirror to reciprocate using interaction between an electrostatic force and a restoring force of a spring. By using the MEMS scanner, it is possible to realize a scanner having characteristics of a high frequency and a large deflection angle compared with the known scanner. Accordingly, it becomes possible to display a high-resolution image.
Here, in order to realize a high-speed MEMS scanner, the mirror should reciprocate about a resonance point. Accordingly, taking light use efficiency or the like into consideration, a system in which a scanning line is scanned from left to right as viewed from a viewer and then a next scanning line is scanned from right to left (both-side scan) needs to be adopted.
On the other hand, a standard of an image signal was decided on the basis of a CRT (cathode ray tube). Accordingly, the image signal is scanned from left to right and then returns to the left within a short time so as to perform the scanning again in the right direction (one-side scanning). Thus, in the case of the MEMS scanner, some data should be displayed by reversing the order of input signals. As a result, it becomes complicated to control a signal.
Further, an electro optic (EO) scanner is considered as a scanning unit other than the MEMS scanner. The EO scanner is a device that changes the propagation direction of light transmitted through EO crystal by applying a voltage to the EO crystal. Thus, in the case of the EO scanner, a scanning angle can be controlled using a voltage. Accordingly, display based on one-side scanning becomes possible, in the same manner as the CRT.
Moreover, in the EO scanner, EO crystal is interposed between a pair of electrodes, and electrons are injected and the electron distribution is biased by applying a voltage to the electrodes. As a result, the distribution of a refractive index also changes due to the Kerr effect and incident light is deflected toward a side corresponding to a high refractive index, such that scanning of light becomes possible. In addition, since a slope of the refractive index distribution inside EO crystal is determined on the basis of the amount of electrons injected, that is, an applied voltage, the scanning angle of light emitted from the EO crystal may be controlled by changing the applied voltage.
In the case of such EO scanner, however, the following problems are still left.
In the EO crystal, a deflection angle of emitted light and the intensity of an electric field generated within the EO crystal are associated with each other, but an electric field intensity required to obtain an effective deflection angle is very high. Therefore, in order to efficiently obtain the large electric field intensity, it is necessary to make a distance between electrodes narrow. However, in terms of the structure of the EO scanner, the distance between electrodes affects an optical path of scanned light. Accordingly, if the distance is set to be too narrow, a laser beam propagating through the EO scanner touches the electrode, which causes a problem that light is not emitted from an emission end surface of EO crystal. Moreover, even if the light is emitted from the emission end surface of EO crystal, it is not possible to obtain a large deflection angle in the EO crystal in which the distance between electrodes is narrow, that is, in the EO crystal having a small thickness. That is, due to the structural problem described above, it is difficult to realize the improvement in efficiency of electric field intensity and an increase in deflection angle at the same time.